This invention is an improvement over the inventions of several earlier applications, to-wit: Ser. No. 08/130,290, filed 1993 Oct. 4, now U.S. Pat. No. 5,338,185, granted 1994 Aug. 16, in the names of Richard W. Henderson and George R. Lightsey; Ser. No. 08/247,925, filed 1994 May 23, now U.S. Pat. No. 5,456,595, granted 1995 Oct. 10 in the name of Richard W. Henderson; Ser. No. 08/297,048, filed 1994 Sep. 30, now U.S. Pat. No. 5,409,370, granted 1995 Apr. 25, in the name of Richard W. Henderson; and Ser. No. 08/365,804, filed 1994 Dec. 29, now U.S. Pat. 5,549,470, granted 1996 Aug. 27, in the name of Richard W. Henderson.